The giant raptor, found in Argentina, measured between 16.5 and 21 feet (5 to 6.5 meters) long, making it one of the largest raptors to roam Earth 70 million years ago, a new study finds.

A dramatic new carnivorous dinosaur that was bigger than a car was unveiled yesterday in public at the Museo Argentino de Ciencias Naturales (the Argentine, or Bernardino Rivadavia, Museum of Natural History) in Buenos Aires, Argentina. The incomplete fossil skeleton was unearthed in Patagonia and was identified as a new species of dromaeosaur, or birdlike dinosaur, a group that includes Velociraptors. Christened Austroraptor cabazai, this specimen is the largest birdlike raptor ever discovered in South America and its discoverers estimate that it lived roughly 70 million years ago.
The fossil bones were found in 2002 in the small Patagonian town of Bajo de Santa Rosa, where this group had previously found several species of herbivorous dinosaurs.

Based on these fossils, the scientists found that Austroraptor cabazai has a long and low snout (figure 1b) with numerous, small pointed teeth — a feature that resembles spinosaurid theropods.

Curiously, it also has very short forearms — dramatically different from other dromaeosaurids and their close avian relatives, all of which have characteristically long arms (figure 1 and figure 2b).

“We don’t know why. It’s a completely different adaptation,” stated paleontologist Fernando Novas, who is the leader of the paleontological team that found the dinosaur. However, he added, this unique feature could “provide a lot of information about the origin of birds” as yet one more link tracing avian evolution from their ancestral relatives.

Figure 2. Phylogenetic relationships of Austroraptor cabazai. (a) Summarized strict consensus depicting the phylogenetic relationships of Austroraptor within Paraves. Austroraptor is deeply nested within Dromaeosauridae and Unenlagiinae. The affinities of A. cabazai were tested using recently published datasets within the broad context of Coelurosauria (Turner et al. 2007) and analysed with equally weighted parsimony in TNT27 (Goloboff et al. 2008). Further phylogenetic data and a complete strict consensus tree are offered in the electronic supplementary material. (b) Phylogenetic tree of paravian coelurosaurs displaying the evolution of forelimb/hindlimb proportions, as measured by the humeral length/femoral length ratio (H/F) optimized as a continuous character using linear parsimony in TNT27. Root of the tree located at the bottom of the figure (marked in grey) and derived branches to the top. The location of nodes and taxa along the horizontal axis represents the H/F ratio (filled circles, measured taxa; open circles, inferred ancestral values; error bars, ranges of possible ancestral values or intraspecific variation for terminal taxa). Grey box encloses dromaeosaurid maniraptorans, exemplified by two silhouettees: a generalized dromaeosaurid with elongated forelimbs, based on Deinonychus (Ostrom 1969), and Austroraptor with reduced forelimbs. See the electronic supplementary material for further explanation. Buitre, Buitreraptor; Deino, Deinonychus; Graci, Graciliraptor; Micr, Microraptor; Sinorn, Sinornithoides; Sinornit, Sinornithosaurus; Sinov, Sinovenator; Unen, Unenlagia [larger view].
DOI: 10.1098/rspb.2008.1554 [free FirstCite PDF].

Austroraptor was unearthed from rock formations that are 70 million years old, indicating this species was one of the last dinosaurs to roam Patagonia before the famous mass extinction occurred at the K-T Boundary at the end of the Cretaceous period, 65 million years ago, killing all non-avian dinosaurs.

“This discovery moves us closer than ever before to the moment of the great extinction,” remarked a pleased Novas.

Paleontologist Fernando Novas with a replica of the dinosaur Austroraptor cabazai at the Argentine Museum of Natural Science Bernardino Rivadavia in Buenos Aires. The dinosaur, of about 70 million years old, was found in the Patagonian province of Rio Negro, SW of Buenos Aires.

The National Geographic Society funded these paleontological explorations and the paper describing this new dinosaur was published by London’s Royal Society. A plastic reproduction of A. cabazai‘s skeleton will be included in an exhibit of Argentine dinosaurs that will debut next year in Europe.

It seems to me that dromaeosaurs were growing larger to fill the niche of the larger therapod dinosaurs like tyrranosaurs. That might explain why this one had smaller arms, like a T-Rex. I don’t know if there were already larger therapods in that time and place, but if not, or if there was enough food to go around, dromaeosaurs could have found success in growing larger and living like tyrranosaurs.

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